Self-organizational Behaviour of Chiral Nematic Liquid Crystals doped with Nanoparticles

Abstract

In the recent past, an increasing interest has been noticed in the nanoparticles doped liquid crystal (LC) composites with a focus on their broader applicability and scientific understanding. The self-assembly of nanomaterials in the soft superstructure of LCs comes naturally due to its anisotropic nature. The use of carbon nanotubes (CNTs) and various other nanoparticles in the nematic and ferroelectric liquid crystals has been shown to improve electro-optic switching behavior, nonvolatile memory effects, diffractive properties and the threshold voltage. Their use is also found in protein biosensors, biocatalysts, bio separators and for drug storage and delivery. Although a lot has been done in this field, the understanding of the manipulation of nanotubes and spherical nanoparticles in the helical superstructures of chiral liquid crystals remains unexplored. In the present study, we have tried to study the effects of nanoparticles and nanotubes doping on the optical, morphological and electro-optic switching behaviour of induced chiral nematic LCs. These investigations are expected to open new avenues in the field of optical memory research in soft materials. Cholesteric liquid crystals (CLC) being chiral, have a helical structure. The pitch of the helix can be compressed or extended by external perturbations. The dispersion and selfassembly of nanomaterials into soft and well-defined functional superstructures is facilitated due to the flexibility and anisotropic nature of liquid crystals. We have induced chirality in a nematic liquid crystal by adding an optimized amount of chiral dopant into it. Functionalized nanomaterials (SWCNTs, MWCNTs, Silver nanoparticles, Aluminum-doped ZnO nanoparticles) were homogeneously dispersed in this medium. The effect of nanoparticle doping on the helical supramolecular structure of induced chiral medium for different concentrations of nanoparticles was investigated in the presence of surface anchoring, electric field, incident radiation and thermal perturbations. Morphological and optical studies of doped materials have shown that nanotubes and nanoparticles produce defects in the CLC phase and break its continuous rotational symmetry. As the performance of the display applications depends on the faster electrically induced Freedericksz transition, the collective reorientation of the liquid crystal director with 8 applied electric field was investigated during the experiment until it gets aligned parallel to the electric field. A significant change in the onset voltage of Freedericksz transition of doped samples as a function of dopant concentration was observed. Multifold enhancement in the PL intensity because of silver and AZO nanoparticle doping highlighted the aggrandized anisotropic behavior of the doped samples. Improved electro-optic response in terms of conductivity and dielectric constant of the carbon nanotubes doped materials was evident from the dielectric investigations of the doped samples. Transmission and photoluminescence behavior of the doped samples were studied to confirm the emergence of the nonvolatile memory in the doped samples.